CLEANUP: errors: also call deinit_errors_buffers() on deinit()

CLEANUP: errors: also call deinit_errors_buffers() on deinit()

Tim reported in issue #1676 that we don't release startup logs if we
warn during startup and quit before creating threads (e.g. -c -f ...).
Let's subscribe deinit_errors_buffers() to both thread's end and
deinit. That's OK since it uses both per-thread and global variables,
and is idempotent.